1. Field of the Invention
An object of the invention is a method for the correction of the measurements of optical density made on a radiographic film. It can be applied, more particularly, to the field of medicine and, in this field it can be applied notably to mammography. The aim of the invention is to enable more reliable discrimination of small objects that can be characterized by their contrast, in also eliminating the disturbing effects due to the non-linearity of the characteristic curve of the sensitivity of the film.
2. Description of the Prior Art
Mammographic examinations are known in medicine. These examinations seek to reveal the microcalcifications which occur within patients' breasts and which may reveal the presence of cancerous tumors. The microcalcifications are small-sized (25 micrometers to some millimeters) calceous formations. The high density of the microcalcifications as compared with the surrounding tissues may enable them to be shown on the photographs despite their small size. It is also known that breast cancer affections can be successfully treated on condition that the treatment is done as early as possible after the appearance of the first signs. It is therefore important to detect the presence of these microcalcifications even though they are hardly visible.
A method for the computer-aided detection of microcalcifications is known, in particular from an article by Heang-Ping CHAN et al, "Computer-Aided Detection of Microcalcifications in Mammograms, Methodology and Preliminary Clinical Study" in the journal Investigative Radiology, September 1988, vol. 22, pp. 664 to 671. In this method, a radiological image, revealed by a radiographic film, is digitized in a known way. Then, on the digital signal got from this digitization, processing operations including, in particular, operations for comparison with a threshold, are carried out in order to make a statistical determination of the presence of clusters of microcalcifications in this radiograph.
One of the essential problems to be resolved then arises out of the fact that the sensitivity of the radiographic film is not linear as a function of the exposure that it has received. This is the integral of the X-radiation received by this film over the time during which this film has been subjected to radiation. The film bears a signal of optical density. The optical density is the blackening, which is pronounced in varying degrees, present in the radiograph after the photographic development of the film. It can be roughly assumed that the sensitivity of the film has three regions. In the first region, the sensitivity is low: the coefficient of correspondence of the variations in difference of exposure received and the associated differences in optical density is low. This first region corresponds to low values of exposure. In a second region, where the values of exposure are higher, the coefficient of correspondence is higher. In a third region, where the exposure is even higher, the coefficient of correspondence is again low.
The difficulty created by this situation in mammography is that the breast is formed by different tissues having different coefficients of attenuation. The variations in optical density are related to the presence and to the proportion of these different tissues. In those parts where the attenuation is the highest, the X-radiation will be more absorbed than in the parts were it is lower. As a consequence, the exposure received by the film directly on the parts where the attenuation is low will be greater than the exposure received directly on the parts where the attenuation is high.
It has been shown that microcalcifications of identical sizes prompt a variation in the exposure of the film that is the same irrespectively of the place in which these microcalcifications are located in the breast. In other words, when measured in terms of exposure, or more precisely in terms of the logarithm of the exposure, these microcalcifications show identical contrasts. Thus, owing to this defect of linearity in the curve of sensitivity, the microcalcifications located in the parts with high or low attenuation will be, to put it schematically, revealed by variations in the signal of optical density on the film that will be relatively smaller than those relating to microcalcifications that are located in those parts of the breast having mean attenuation (and that correspond to the exposed parts in an intermediate way: i.e. with the greatest sensitivity of the film).
To resolve this problem it has been discovered, in the invention, that the curve of sensitivity of the film has to be established in such a way that each value of optical density measured on the radiograph must be assigned an equivalent value corresponding to the logarithm of the exposure. This is achieved by a corrective function. In practice, this corrective function is the reverse of the function characterized by the characteristic curve of sensitivity of the film. In this way, the effects of this non-linearity are removed. It is therefore necessary to read the characteristic curve of sensitivity of the film.
There is also a known method by which, after the mammographic examination, phantoms of known radiological densities are placed on the film. An additional printing of the film is done by means of an apparatus called a sensitograph. The exposure of the parts of the film at the position of these phantoms is then known. The optical density values of the film, when it has been subsequently developed, may be assessed. Using these assessments of optical density and this knowledge of the exposure at the position of the phantoms, it is possible to analyze the radiograph by comparison. However, this technique has the drawback of being cumbersome to implement and, in practice, it is not implemented. What happens essentially is an instinctive human visual correction, for the practitioner then works on a radiograph and not on a digitized image. When this technique is not implemented, the correction is obtained by the experience of previous studies carried out by the practitioner on films of the same grain, and the same theoretical sensitivity, that have been subjected to similar exposures or are supposed to have been developed in the same way. Furthermore, this technique is applicable only if the characteristics of the shooting and the characteristics of the radiograph itself are known. This technique, therefore, cannot be applied to a radiograph of unknown origin or a radiograph that has no traces of phantoms; and yet this radiograph may be one that has to be used because it was taken at an earlier period and needs to be examined in order to assess the prior presence of these microcalcifications.
In the article cited, since the authors were unable to isolate the microcalcifications by a comparison with a single contrast value on the entire radiograph, they made use of an adaptive thresholding method. This is a standard technique in image analysis. The method consists in relating the thresholding value at one point to the information contained in the geographical vicinity of this point. In this specific case, they condition the thresholding by the value of the mean square deviation on the original image or on the spatially filtered image (the technical description is ambiguous) in a window with a size of 51 pixels by 51 pixels, i.e. a 5.1 mm.times.5.1 mm window since their images are digitized at 100 micrometers by 100 micrometers per pixel. These authors present an approach that provides for more efficient thresholding but does not seek to correct the effect of the variation in sensitivity. The difference between their approach and that of the invention is important from this point of view. The drawback of the method of this article is that the background of the image plays too great a role in the correction.
In an improvement, the present invention is aimed at overcoming these drawbacks in proposing a different technique by which the slope of the characteristic curve of a radiographic film is truly obtained even, in this case, without having any special prior information on the type of film, its conditions of exposure or its conditions of development. No presence of phantoms is necessary, in principle, for the invention. It has been discovered, in the invention, that the noise present on the film could enable an estimation of the characteristic curve of sensitivity of the film which represents the transfer function between the luminance of the X-photons that have imprinted the film and the blackening of the film resulting from this imprinting. The noise constitutes an exploratory phenomenon, at every dot of the film, of the sensitivity of the film at the place where this phenomenon occurs. It therefore suffices to measure the revealing of this noise. The characteristic curve is deduced therefrom.
There is a known method, described in an article by G. T. Barnes and D. P. Chakraborty, "Radiographic Mottle and Patient Exposure in Mammography" in the journal Radiology, Vol. 145, No. 3, pages 815-821, December 1982, in which the characteristic curve is related in analytical form to the noise measured in the image. But the conclusion drawn therefrom is that the exposure of the apparatus to X-rays must be optimized. This is not possible when dealing with acquired radiographs. Above all, this does not teach us how to compute the characteristic curve itself but, on the contrary, assumes that it is known.